This happens to players all the time and forces people to change things up when facing people of the same color. Sometimes it can be used strategically, but you have to know the other person's strategy really well in order to do something like that.

Watch out also for "Character backlash" which happens when there's a character card on the field and you or your opponent tries to play the same character card. It'll get sent immediately to the cemetery. Watch out too, even if the character card is closed, it'll still count as being there and backlash.

Both going for backslash and avoiding it can be used as a strategy, but of course you have more chance to use this if both you and your opponent are using modded starter files. I used to have haste and eagle soldier from falkow in my gowen deck too for fun and for being able to backslash falkow players a few times, generally hurting their early game tactics and to avoid gowen-backslash, since this sphere has only about 2-3 useful way to start a game if it doesnt have many extra cards included. Later with my falkow deck i went for backslashes if i had a few returns to further ruin my opponents strategies or if i had 1-2 ruby carbuncle SS to get back those lost third cards from each unit (i suppose this could be used by anyone who has cards to bring back units from cementery).

Something must be wrong with me. I'm giving advices to someone i should kill.